The present invention relates to a charger for lithium secondary batteries.
In portable electronic apparatus, such as information devices, communications devices, receivers, and video/music recorder/players, and transportation apparatus, such as electric vehicles, their throughput, display ability, power performance, driving time, etc. have recently been required to be improved. Therefore, electrochemical devices used in such apparatus have been required to have higher capacities, higher outputs, and improved cycle life. Such requirements have lead to the developments of electrode materials for realizing batteries with high energy density, and to the developments of charge systems for maximizing the performance of high energy density batteries to make such apparatus operate effectively.
With respect to electrode materials for such lithium secondary batteries, carbon materials, such as graphite, have already been put to practical use as negative electrode active materials, because they are superior in reversibility and reliability. However, since the practical capacity of graphite has become close to the theoretical capacity thereof, research into electrode materials having higher capacities is being carried out. For example, Japanese Laid-Open Patent Publication No. Hei 07-29602 and Japanese Laid-Open Patent Publication No. 2002-352797 propose the use of elements such as silicon (Si) and tin (Sn) and alloys including such elements, which are theoretically expected to offer capacities significantly higher than conventional electrode materials.
An electronic apparatus equipped with a conventional charge system is schematically illustrated in FIG. 5.
An electronic apparatus 50 of FIG. 5 includes a power source 51, an electrochemical device 52, a load 53, a voltage comparator 54 which measures the voltage of the electrochemical device 52 relative to threshold voltage Vth, a charge controller 55, a charging circuit 56, a current measuring means 57 which measures the current flowing through the electrochemical device 52, and a temperature detecting means 58 which measures the temperature of the electrochemical device 52. The charge system is composed of the voltage comparator 54, the charge controller 55, the charging circuit 56, and the current measuring means 57 and the temperature detecting means 58. Also, the threshold voltage Vth is applied by a reference power source 59.
Such a charge system detects the voltage of an electrochemical device, such as a lithium secondary battery or a nickel-metal hydride battery, and determines the remaining capacity of the electrochemical device in consideration of factors, such as current and temperature upon the voltage detection.
With such a conventional charge system, when an electrochemical device is fully charged, the charging of the electrochemical device is stopped. When the voltage of the electrochemical device drops below the lowest operating voltage of the apparatus, the discharging of the electrochemical device is stopped, and the electrochemical device is then fully charged. This ensures that the electrochemical device constantly has a maximum remaining capacity before the use of the apparatus, thereby making it possible to supply electric power stably. Such stable power supply enables the apparatus to have a maximum driving duration.
However, according to such charge/discharge method in which an electrochemical device is charged up to 100% remaining capacity and is discharged down to the lowest operating voltage of an apparatus powered by the electrochemical device, repetitive charge/discharge cycles cause the battery capacity to drop significantly upon full charge. That is, a problem of cycle life deterioration occurs. In order to solve this problem, there have been proposed methods that employ two end-of-charge voltages: a first end-of-charge voltage which maximizes the charge capacity of an electrochemical device; and a second end-of-charge voltage which is lower than the first end-of-charge voltage. According to these methods, an electrochemical device is charged up to one of the end-of-charge voltages, depending on the circumstances (e.g., see Japanese Laid-Open Patent Publication No. 2002-218668 (page 9, FIG. 1) and Japanese Laid-Open Patent Publication No. 2002-359008).
However, how to determine such voltages and the deterioration mechanism of electrochemical devices are not clearly described in Japanese Laid-Open Patent Publication No. 2002-218668 and Japanese Laid-Open Patent Publication No. 2002-359008. Thus, when such a charging method is used to charge an electrochemical device, end-of-charge voltages must be determined by trial and error. When the thus-determined end-of-charge voltages are used as reference voltages in performing charge/discharge control, the set end-of-charge voltage may be lower than preferable end-of-charge voltage, since the deterioration of electrochemical devices is not completely analyzed. In this case, the electrochemical device is not charged fully, so there is a problem in that the operating time of an electronic apparatus, a transportation apparatus, or the like becomes shortened.
On the other hand, if the set end-of-charge voltage is higher than the preferable end-of-charge voltage, not only battery life but also the reliability, safety and maintainability of the charge/discharge system deteriorate.
Further, in the charging method of Japanese Laid-Open Patent Publication No. 2002-218668, the second end-of-charge voltage is employed when a battery incorporated in a notebook PC is trickle-charged by AC power through an AC adapter to make up for the self-discharge of the battery. Therein, by setting the second end-of-charge voltage to a voltage lower than the first end-of-charge voltage, the deterioration of the battery in a high-temperature environment is prevented. In this case, it appears that the second end-of-charge voltage, which is lower than the first end-of-charge voltage, is determined on the assumption that the decomposition of the electrolyte is prevented even under a high-temperature environment. The second end-of-charge voltage is not determined in consideration of the deterioration in cycle life.
Meanwhile, the cycle life of a battery may deteriorate when a material that is expected to provide a high capacity, such as Si, absorbs a large amount of lithium during charging. The reason is described below.
When a negative electrode contains lithium-containing silicon represented by the composition formula LixSi, the molar ratio x of Li to Si, which represents the charge/discharge depth of the negative electrode, is correlated to charge/discharge cycle life.
As the molar ratio x of Li to Si increases, the lithium-containing silicon undergoes phase changes at predetermined molar ratios x, and it is known that such phase changes cause the following five phases: x=0 (Si: cubic); x=1.71 (Li12Si7: rhombic); x=2.33 (Li14Si6: rhombohedral); x=3.25 (Li13Si4: rhombic); x=4.4 (Li22Si5: cubic). In the lithium-containing silicon, the higher the value x is, the higher the theoretical capacity is.
The occurrence of the above-mentioned phase changes due to changes in the value x results in changes in the volume of the lithium-containing silicon. For example, when x=4.4, the volume of the lithium-containing silicon expands approximately 4.1-fold, compared with the volume when×=0. Thus, if the full charge and full discharge of a battery including lithium-containing silicon as a negative electrode active material are repeated, the volume of the lithium-containing silicon changes greatly. Upon repetition of such charge and discharge, the lithium-containing silicon becomes unable to accommodate such volume changes, so that the lithium-containing silicon itself becomes broken, thereby resulting in a deterioration in the cycle life of such a battery.
Further, if the value x increases to cause a large expansion of the negative electrode, the current collecting performance of the lithium-containing silicon lowers. As a result, a battery with a negative electrode including lithium-containing silicon has a problem of degradation of the actual capacity of the battery.
In view of the above, it is therefore an object of the present invention to provide a charger that is capable of charging a lithium secondary battery including lithium-containing silicon as a negative electrode active material such that the battery has a high capacity, or a higher capacity if necessary, without accelerating the deterioration in cycle life, and an electronic apparatus equipped with such a charger.